Dimethyl ether, which has the formula CH3—O—CH3, is a compound whose industrial synthesis has many applications.
Dimethyl ether can especially be used as a precursor for the synthesis of various compounds of interest, such as low molecular weight olefins, methyl acetate or dimethyl sulfate. It can also be used as such, for example as a propellant.
More specifically, dimethyl ether has been proposed as an alternative fuel to advantageously replace petroleum derivatives. It is a readily liquefiable gas (its boiling point is −25° C.) which has a cetane index comparable to that of diesel. In addition, dimethyl ether has the advantage of being much less polluting than petroleum derivatives, especially in terms of sulfur oxide, nitrogen oxide and soot emission, which makes it more compatible with recent developments in legislation relating to exhaust gases. For further details regarding the advantages of using dimethyl ether as an alternative fuel, reference may be made especially to the article by Semelsberger et al. in Journal of Power Resources Vol. 152(1), pp. 87-89 (2005).
At the present time, dimethyl ether is already being used in the domestic fuel sector to replace liquefied gases obtained from petroleum (butane and propane), and it is being considered as a fuel for the large-scale production of electricity, especially in India where, on its own, it should provide half the electricity produced in 2010.
A conventional production technique consists in synthesising dimethyl ether from a mixture of CO and H2 (so-called “synthesis gas” mixture), which is reacted with a suitable catalyst (generally based on metallic oxide), for example according to the methods described by T. Ogawa et al. in Journal of Natural Gas Chemistry Vol. 12, pp. 219-227 (2003) or alternatively in documents GB 1 398 696, U.S. Pat. No. 4,177,167, GB 2 099 327 or GB 2 093 365.
Another technique for synthesising dimethyl ether, which has been developed more recently, consists in preparing that compound by dehydrating methanol on an acidic catalyst according to the following reaction:2CH3OH→CH3—O—CH3+H2O
This reaction has been described especially by K. W. Jun et al. in Bulletin Korean Chemical Society Vol. 24, p. 106 (2003).
The above-mentioned methanol dehydration reaction usually employs solid catalysts based on gamma aluminium or modified gamma aluminium, of the type described, for example, in documents U.S. Pat. No. 4,560,807, EP 270 852 or GB 403 402. Such catalysts have a disadvantage, namely that, given their hydrophilic nature, they are deactivated in the presence of water, which prevents their use especially for the conversion of methanol obtained from biomass unless laborious and costly methanol pretreatment processes are carried out.
In order to remedy the disadvantages encountered with catalysts of the gamma aluminium type, more specific catalyst systems have been proposed.
In WO 04/74228 there is described a dual catalyst system which makes use of a hydrophilic acidic catalyst, which allows to convert methanol into dimethyl ether, coupled with a hydrophobic acidic catalyst, which ensures that the methanol is maintained in a state of dehydration. Although this system exhibits good synthesis yields, it is found to be rather difficult to carry out.
In order to catalyse the methanol dehydration reaction the use of zeolites has also been considered, especially MFI-type zeolites, for example zeolite ZSM-5, which are relatively stable in the presence of water and the acidity of which can be modified, especially by impregnating them with a solution of sodium salts, which allows relatively good yields to be obtained.
Nevertheless, in connection with these various advantages, the use of zeolites of the type of the zeolite ZSM-5 is found to be not entirely satisfactory in practice, in particular when it is desired to use the dehydration reaction on a large scale.
In fact, it should be emphasised that such catalysts have a major disadvantage, namely that they are not stable over time. More precisely, it is found that, when catalysts constituted of zeolite of the ZSM-5 type are used to carry out the conversion reaction of methanol to dimethyl ether, a very rapid loss of activity of the catalyst is generally observed, which manifests itself in concrete terms in a very marked reduction in the methanol conversion over time, the loss of activity generally being observed after very short reaction times of the order of several hours at most, typically after from 2 to 6 hours of reaction.
It seems that the loss of catalytic activity observed when catalysts constituted of zeolites such as ZSM-5 are used can be explained at least partly by a so-called coking phenomenon, namely the gradual deposition of carbon within the structure of the zeolite. Also to be taken into account are a possible alteration of the zeolite structure by the water vapour formed during the conversion reaction of methanol to dimethyl ether, as well as the potential influence of the regeneration of the structure of the catalyst in the air. Moreover, the conversion reaction is exothermal, which is liable to form hot spots on the catalyst which are capable of promoting the above-mentioned phenomena.
Furthermore, catalysts made of zeolites such as ZSM-5 have another disadvantage: they are in most cases in the form of powders, which are relatively difficult to use and lead especially to considerable losses of potential within the reactors, preventing their use on an industrial scale. In order to avoid that problem, it has been proposed to form the zeolite powders into macroscopic solids, especially by extrusion in the presence of inorganic binders of the silicon oxide or aluminium oxide type. However, such a solution is found to be unsatisfactory especially insofar as the binding agents used render part of the zeolite inaccessible, and they are additionally liable to induce undesirable secondary reactions.